Before the dma transfer, pci_map_sg will map the memory space covered by
sg_table,
and it will flush and invalidate cache indexed by that memory space.
After the dma transfer, pci_unmap_sg also needs to be called to flush and
invalidate
the same cache. But I do not know why the second flush will be demanded.
I think that in the interval between the two flush, there would be nothing to
access
the memory covered by the dma. But it is not the case.
I want to know what can access that memory and I need some help.

In 2.4, memcpy's prefetch may (and, in practice, do, no smiles, it cost
me a lot of time to realize) access that memory. I though it has been
fixed in 2.6 someday.